The absolute most accurate way to determine the gear ratio is to count the teeth on the ring gear, count the same on the pinion year and divide the ring gear number by the pinion number.

Which is great if you have it apart, but not much help determining what it is while sitting in the driveway. Easiest way to determine is to jack up the back and turn the rear wheel and note how many revolutions it makes compared to the revolutions of the driveshaft.

Depends on what is available for your axle. There may or may not be taller factory gears. There was NOT for my '46 so I swapped the whole rear end for a late model Dana 80. Stout and plenty of gearing options plus disk brakes. Post war trucks may have a few more choices but it will take a little homework to discover what factory stuff is out there. There are also some custom gear services that make wider ranges for some makes & models (depends on popularity).

Happy hunting and best of luck.

BTW...I was only guessing at the 45-50 mph. You need to run all your numbers through a gear calculator to determine what speed at what RPM.
I use the set below. The one at the very bottom of the page is excellent and you can play "what if" with any of the numbers.

About the rear bumper of your Blue Bird, if it should be for sale.... Well, dang... I was in Medford two weeks ago! But I'm only a tad over 300 miles south of you. And I may visit my friends in Medford again this spring.

Interesting project you are working on!
Now... your old bus must have a stick shift? Since you are accustomed to that... might you want a stick shift with the new setup? If so, you can get stick transmissions with an overdrive ratio in the top gear.

cool bus spiritjohn.. i will be watching your build for sure.. in my 1941-47 white bus i used a 70's international rear end i got out of a junk yard for 100.00.. i dont remember the exact gear ratio with 11R 22.5 tires it runs 65mph at 1900 rpm ..i usually get 14-16 mpg if i stay under 60mph which is very hard to do.. i have air brakes on my bus so not sure if you would want to go that route with yours